U.S. Pat. No. 4,482,833 to Weinert et al. discloses a method of depositing thin films of gold having a high degree of orientation on surfaces previously yielding only unoriented gold by sputtering a layer of glass over the surface of the material followed by depositing a layer of oriented gold over the layer of glass. The additional step of depositing a layer of piezoelectric material over the layer of oriented gold is included to provide piezoelectric material having good orientation due to the oriented gold. The transducer described possesses a layer of glass deposited over a material which previously provided unoriented gold followed by a layer of gold, a layer of piezoelectric material and a top conductive electrode to form a transducer wherein the piezoelectric material has a high degree of orientation. While this reference discloses generally many of the elements of the present invention, it does not recognize the specificity of the thicknesses of the piezoelectric layer and the electrode material layer as is required by the present invention.
U.S. Pat. No. 4,749,900 to Hadimioglu et al. discloses a multi layer acoustic transducer wherein the thickness of the piezoelectric layer is approximately one half the wave length of the acoustic operating frequency. The reference does disclose the use of gold as the top and lower electrodes. But this reference does not disclose the required thickness ratio of a suitable electrode material and piezoelectric layer in order to improve grey scale printing resulting from the use of the improved transducer.
U.S. Pat. Nos. 4,006,444, 4,430,897, and 4,267,732 all to Quate and U.S. Pat. No. 3,774,717 to Chodorow disclose imaging apparatuses that included a transducer coated with a thin layer of gold. Neither the specific structure or use of the present invention is disclosed in these references.
The standard acoustic ink print head embodies a substrate having an acoustic wave generating means which is generally a planar transducer used for generating acoustic waves of one or more predetermined wave lengths. The wave generating means is positioned on the lower surface of the substrate. The transducer noted above is typically composed of a piezoelectric film such as zinc oxide positioned between a pair of metal electrodes, such as gold electrodes. Other suitable transducer compositions can be used provided that the unit is capable of generating plane waves in response to a modulated RF voltage applied across the electrodes. The transducer will be generally in mechanical communication with the substrate in order to allow efficient transmission of the generated acoustic waves into the substrate.
Generally an acoustic lens is formed in the upper surface of the substrate which is used for focusing acoustic waves incident on its substrate side to a point of focus on its opposite side. The acoustic lenses (whether spherical lenses or Fresnel lenses)are generally adjacent to a liquid ink pool which is acoustically coupled to the substrate and the acoustic lens. By positioning the focus point of such a lens at or very near a free surface of the liquid ink pool, droplets of ink can be ejected from the pool.
In the past, to achieve grey levels in acoustic ink printing, two approaches have been identified:
In the first approach, changing the length of the RF (and hence the acoustic) burst increases the droplet size by up to two times from its diffraction-limited minimum diameter of approximately one wave length; the second approach is to vary the number of droplets that are deposited per pixel.
The present invention generally relates to a novel method and means for achieving variable grey levels in acoustic ink printing. More particularly, it relates to an acoustic ink printer having a piezoelectric transducer constructed so that the transducer can generate a sound wave at either its fundamental resonance frequency or at the second harmonic thereof, thereby enabling the ejection of droplets of substantially different diameters.